SOFCs Components: anodes

Transcription

1 Laurea Magistrale in Scienza dei Materiali Materiali Inorganici Funzionali SOFCs Components: anodes Prof. Antonella Glisenti - Dip. Scienze Chimiche - Università degli Studi di Padova

2 Bibliography 1. N.Q. Minh, T. Takahashi: Science and technology of ceramic fuel cells Elsevier J.-H. Lee et al. Solid State Ionics 148 (2002) P.R. Slater, J.T.S. Irvine Solid State Ionics 124 (1999) P.R. Slater, J.T.S. Irvine Solid State Ionics 120 (1999) J. Canales-Vázquez, S.W. Tao, J.T.S. Irvine Solid State Ionics 159 (2003) J.C. Ruiz-Morales et al. Nature 439 (2006) Y.-H. Huang et al. Chem. Mater. 21 (2009)

3 Anode: requirements Functions: To provide reaction sites for the electrochemical oxidation of the fuel Requirements: Stability chemical, morphological, dimensional stability at the fuel atmosphere (inlet and outlet) and at the operating and fabrication temperatures (no disruptive phase transformation) Electronic (Mixed) conductivity in the fuel atmosphere (at the operating temperature) to minimize ohmic losses (constant with P O2 changes) Compatibility chemical compatibility with the other cell components Thermal expansion must match (from RT to the operating and fabrication temperatures) that of other components; thermal coefficient stable in the reducing atmosphere Porosity high porosity to allow gas transport to the reaction sites Catalytic activity High catalytic activity to low polarization for electrochemical oxidation of fuel (poison tolerance)

4 Anodes Spacil (1970) = a composite of nickel and YSZ particles can provide a stable and highly active anode; good mechanical properties and geometric stability Composition, particle sizes, manufacturing method Drawbacks: Sensitivity to sulfur (1 ppm H 2 S at 1000 C, 50 ppb at 750 C) and other contaminants (HCl irreversible > 200 ppm) Oxidation intolerance: the anodes must be kept under reducing conditions at all times Thermal expansion coefficient substantially higher than the electrolyte and cathode. Mechanical and dimensional stability problems in anode-supported designs Poor activity for direct oxidation of hydrocarbons and propensity for carbon formation (copper ceria anodes).

5 Nickel/YSZ Cermet properties Functions: Ni = low cost active material; YSZ = To support of the nickel-metal particles; To inhibit Ni particles coarsening and maintain a porous structure To provide a thermal expansion coefficient acceptably close to those of the other cell components Properties (reducing atmosphere):

6 Nickel/YSZ Cermet electrical conductivity S-shape: electrical conductivity of composites < 30 vol% Ni conductivity of the cermet is similar to that of YSZ (ionic conduction path through the YSZ phase) > 30 vol% electronic conduction (decrease with temperature increase, activation energy (5.38 kj/mol) similar to that of Ni); conductivity depend on reduction > 30 vol% > surface area < coverage (at the same Ni content) < particle-to particle contact < conductivity Conductivity of Ni/YSZ cermet at 1000 C as a function of Ni content Temperature dependence of conductivity of Ni/YSZ cermet

7 Nickel/YSZ Cermet preparation In most cases 1) NiO and YSZ; 2) NiO reduced in situ (porosity increases) Anode microstructure after air firing (A) and hydrogen reduction (B) Relationship between air firing and hydrogen reduced porosity

8 Nickel/YSZ Cermet electrical conductivity conductivity depends on reduction time Anode conductivities as a function of time during NiO reduction

9 Nickel/YSZ Cermet: Morphology and Performance In the conventional powder mixing process the anode morphology depend on the starting powder properties The anode overpotential depends on morphology Relationship between nickel/ysz anode overpotential and particle size ratio of starting powders

10 Nickel/YSZ Cermet: Morphology and Stability Volume change under operating conditions a continuous YSZ network formation is necessary to support Ni particles and avoid morphology and dimensional changes fabrication conditions (preparation procedure, temperature, ) and starting materials (particle dimensions, Ni content, ) Relationship between anode volume change and YSZ content with various YSZ particle sizes

11 Nickel/YSZ Cermet stability: Morphology and Performance Ni sintering: < surface area < conductivity < cell performance Effect of Ni sintering on cermet anode polarization

12 Nickel/YSZ Cermet stability: Morphology and Performance Effect of coarsening of the Ni/YSZ on the polarization of the anode: N p = number of pores per unit area ρ = electrolyte resistivity r 0 = initial particle radius k r = proportionality constant t = time L = electrode thickness At initial stages of coarsening (t = 0): r = pore radius Z = interfacial resistance between Ni and YSZ η = (1/N p )(ρψ) 1/2 coth(ρl/ψ) 1/2 At long period of time (t very large): Anode polarization will increase rapidly at the beginning and continue to increase as long as the driving force for Ni sintering remains significant

13 1. High electrical conductivity to reduce the ohmic loss. 2. Enough electrochemical activity to reduce the activation polarization 3. Proper microstructural condition to reduce the concentration polarization High Ni content = high electrical conductivity, instability of microstructure due to Ni coarsening. Highly porous composite = lower concentration polarization, improper mechanical and electrical properties.

14 Quantitative analysis of microstructure and its related electrical property of SOFC anode, Ni YSZ cermet Raw materials: NiO, YSZ (Average particle size = 2µm) Mixing (ball milling 24h acetone/isopropylalcohol) M2 = 13% Ni M3 = 20% Ni M4 = 28% Ni M5 = 37% Ni M6 = 47% Ni M7 = 58% Ni M8 = 70% Ni Spray drying Sieving (< 150 microns) Prepressing Reduction (1000 C, H 2 ) Debinding & sintering ( C)

15 Phase analysis M8 = 70% Ni M7 = 58% Ni M6 = 47% Ni M5 = 37% Ni M4 = 28% Ni M3 = 20% Ni M2 = 13% Ni NiO and YSZ well ordered phases all NiO diffraction peaks disappeared after reduction: All NiO YSZ composites were successfully transformed to Ni YSZ cermets increasing amount of Ni content XRD patterns of anode composite (a) before and (b) after the reduction.

16 Density analysis The appropriate porosity level of Ni YSZ cermet for SOFC application is around 40%. By considering that about 41.1% of initial volume of NiO is transferred to pores during NiO reduction to Ni metal, porosity of sintered NiO YSZ composite as around 10 20% is required: 1400 C, 3 h 1500 C, 30 min 1400 C, 3 h: compositional variation due to the evaporation of NiO at 150 C C 3 h 1500 C 30 min 1400 C 3 h 1400 C 30 min Green density 1400 C 30 min 1400 C 3 h 1500 C 30 min 1500 C 3 h

17 Pore size and composition NiO20 as NiO content increases, the pore size of the composite looks bigger even though the overall porosity is hardly different with each other More pronounced grain growth occurred in the NiO phase due to the difference of sinterability between NiO and YSZ phase at 1400 C NiO60 NiO40 NiO80

18 Porosity and treatments the relative density decreases as the NiO content is increased because the higher NiO content the more the oxygen extraction during the reduction which caused the increase of porosity the porosity did not reach theoretically calculated value. The deviation of the measured porosity against theoretical value became greater at higher Ni content. This is due to the coarsening of the Ni phase during heat treatment, which influences the porosity Sintering 1400 C 3 h Reduction in H C 30 min Theoretical open porosity Reduction in H C 30 min Sintering 1400 C 3 h

19 Porosity and Ni content Ni Brightness inverted The image analysis method is valid for Ni/morphology investigation Pore Comparison of Ni contents from image analysis and theoretical calculation YSZ Etched HCl Micrographs of Ni-YSZ composite (M8) after reduction

20 The particle growth The average particle size is larger at higher Ni fractional composition: > contact probability = > grain growth Overall particle size of Ni larger than YSZ: YSZ grain growth mainly occurred during sintering Ni Grain growth occurred at a greater rate during reduction. pore perimeter increases with Ni content: Microstructural evolution controlled by Ni coarsening The increase of pore perimeter also due to the complex pore shape Effects of Ni contents on (a) grain size of Ni and YSZ and (b) pore perimeter.

21 C α = contiguity degree of contact of the α-phase in a three-phase mixture. N L = number of contact point in a unit length, α = Ni β = YSZ γ = pores Images Line graphs

22 Contiguity and Composition Contiguity between the same phases is proportional to the composition while contiguity between different phases shows rather complicated dependence. For contiguity between Ni and YSZ, maximum point is located at around 40 vol % of Ni, in contrast to the expectation (50 vol %). It is due to the microstructural evolution, Composition and microstructural evolution are both fundamental for the contiguity of different phases Contiguity of (a) Ni Ni and YSZ YSZ and (b) Ni pore, Ni YSZ, YSZ pore.

23 Interfacial Area and Composition the interfacial area between the same phases was proportional to the content of that phase The interfacial area between different phases has a different trend than contiguity. Maximum point at different positions: Ni pore = 35 vol % Ni YSZ = 50 vol %: The effect of Ni coarsening. Variation of (a) Ni Ni and YSZ YSZ grain boundary area and (b) interfacial area of Ni pore, Ni YSZ, YSZ pore.

24 Interfacial Area: the growing phases I - YSZ forms a rugged skeleton. Ni coarsening occurred preferentially to the direction of pore II Ni coarsening also occurs to the YSZ phase Variation of interfacial area of Ni pore, Ni YSZ, YSZ pore with Ni content. III Neither YSZ nor pore can control the Ni coarsening and all the interfacial areas were decreased.

25 Electrical conductivity in composites General Effective Medium (GEM) theory to calculate the electrical conductivity of composites t is the exponent for GEM equation and f and f c = volume fraction and critical volume fraction of the poor conductive phase (YSZ), respectively. GEM theory presumes rather ideal situation like similar sizes, spherical and isotropic shapes of particles. Porosity over 40%: Ni YSZ cermet not anymore a twophase composite. Variation of electrical conductivity as a function of Ni contents at 1000 C.

26 Electrical conductivity, morphology, composition electrical conductivity of the composite is controlled by Ni when the contiguity of Ni Ni was larger than around 0.2 and the contiguity of YSZ YSZ is smaller than 0.2. The proper composition to fulfill the previous necessary conditions for anode = Ni content is around vol %. Variation of electrical conductivity as a function of contiguity at 1000 C.

27 Nickel/YSZ Cermet stability: Morphology and Performance To avoid Ni sintering a continuous YSZ network formation is necessary to support Ni particles and avoid morphology and dimensional changes Ni particle size distribution: > width > sintering > wetting < sintering

28 Fabrication techniques to minimize Ni sintering pyrolysis of metallic soap slurry (to deposit fine YSZ particles on the surface of NiO) controlled microstructure and improved adhesion and morphological stability Micrograph of anode prepared by pyrolysis of metallic soap slurry Preparing a slurry of NiO in a Zr and Y octylate solution and firing to polymerize and decompose the organometallics to form YSZ on the NiO particles

29 Fabrication techniques to minimize Ni sintering CVD + EVD (chloride precursors: ZrCl 4, YCl 3, O from NiO) liquid phase synthesis with YSZ sol to deposit well-dispersed Ni on a MgO-YSZ support (long term stability and suppressed grain growth) Microstructure of Ni/MgO-YSZ anode prepared with YSZ sol

30 Electrochemical vapor deposition Ni/YSZ by slurry coating followed by electrochemical vapor deposition of YSZ The process involves growing a dense layer of electron- or ionconducting oxide on a porous substrate at elevated T and reduced P Stage I: formation of the oxide in the pores of the porous substrate by direct reaction of metal chloride with H 2 O the oxide closes the pores and no further direct reaction occurs; complete pore closure is assured MeCl y + y/2 H 2 O = MeO y/2 + yhcl Stage II: growth of the oxide over the closed pores (Wagner oxidation) H 2 O is reduced at the water vapour side to produce oxygen ions that diffuse through the film to the metal chloride side Growth in the direction of the chloride gas phase side (oxygen ions are more mobile than metal cations) y/2 H 2 O + y/2 V O + ye - = y/2h 2 + y/2 O x O MeCl y + y/2 O x O = MeO y/2 + y/2 Cl 2 + y/2v O +ye-

31 Nickel/YSZ Cermet chemical interaction Ni/YSZ anode has negligible chemical interaction with YSZ electrolyte and LaCrO 3 interconnect at T < 1000 C; at higher temperatures poor conducting phases (NiCrO 4 ) form > In cofiring NiO/YSZ anode laminated with LaCrO 3 interconnect liquid phases present in the interconnect tend to migrate into the electrode forming a reaction layer at the interface (1400 C 1 h: 100 µm thick diffusion layer) Elemental distribution in cofired anode (NiO/YSZ)/interconnect (doped LaCrO 3 )/cathode (Srdoped LaMnO 3 )

32 Nickel/YSZ Cermet thermal expansion Thermal expansion coefficient increases with increasing Ni content Use of additives (to electrolyte, to the anode) to increase tolerance of stresses and to improve anode thermal expansion match Thermal expansion coefficients of YSZ Thermal expansion coefficient of cermet anode as a function of Ni content

33 1. Materials less susceptible to coking or S poisoning 2. High electronic conductivity/mixed conductivity 3. Low reducibility of the anode (such anodes should contain transition metals that are stable against complete reduction under solid oxide fuel cell operating conditions).

34 Other materials Cobalt/Ca-doped zirconia: Co: high S tolerance, > oxidation potential, > cost Ru/YSZ: higher melting point (2310 C) = better resistance to particle corasening, high catalytic activity for steam reforming, negligible carbon deposition Mixed conductors (ionic-electronic): reaction over the entire electrode/gas interfacial area) = polarization losses significantly lower. ZrO 2 -Y 2 O 3 -TiO 2 (15% mol TiO 2, 12% mol Y 2 O 3 ; 9.3% mol TiO 2, 10% mol Y 2 O 3 ) Doped Ceria particle with highly dispersed metal catalysts on the surface (significant catalytic activity at reduced temperatures).

35 SrTiO 3 Relatively difficult to reduce Enhancements in the conductivity through suitable doping A rich wealth of defect chemistry is accessible, with samples containing cation vacancies, anion vacancies, and anion excess being investigated.

36 (a) The perovskite (SrTiO 3 ) structure Spheres =A cations, Octahedra = BO 6. Tungsten Bronze Structure The tetragonal tungsten bronze structure can be obtained from the perovskite by rotation of some of the TiO 6 octahedra: 40% of the large cation sites are increased in size from tetracapped square prisms to pentacapped pentagonal prisms, 20% remain unchanged, and the remaining 40% of the sites are decreased in size (C site). If only the former two sites are occupied, then the composition is A 0.6 BO 3. (b) the tetragonal tungsten bronze (Sr 0.6 TiO 3 ) structure. Spheres=A cations, Octahedra=BO 6.

37 Doped SrTiO 3. Doping with Nb (for Ti) or La (for Sr), with charge balance by the introduction of vacancies, oxygen Doping with La: Sr 1-3x/2 La x TiO 3 (0 x 0.6) - ( 7 S cm -1 ) Doping with Nb: Sr 1-x/2 Ti 1-x Nb x O 3 (x 0.4) - ( 10 S cm -1 ) respectable conductivities at elevated temperatures under reducing conditions stability under both oxidizing and reducing conditions. poor oxide ion conductivity (low levels of oxide ion vacancies) Related Tungsten Bronze Phases, (Sr/Ba) 0.6 Ti 0.2 Nb 0.8 O 3 by doping with Nb to higher levels (Ba, Sr, Ca, La) 0.6 M x Nb 1-x O 3 (M=Ni, Mg, Mn, Fe, Cr, In, Sn).

38 Nb based tetragonal tungsten bronzes (Sr 1-x Ba x ) 0.6 Ti 0.2 Nb 0.8 O 3 Sr 0.6-x La x Ti 0.2+x Nb 0.8-x O 3 (Sr 0.4-x Ba x )Na 0.2 NbO 3 (Ba 1-x Ca x ) 0.6 Ti 0.2 Nb 0.8 O 3 Ba 0.5-x A x NbO 3 (A = Ca, Sr) Ba 0.3 NbO 2.8 Solid state synthesis from SrCO 3, CaCO 3, La 2 O 3, Na 2 CO 3, TiO 2, Nb 2 O 5 Intimately mixed and heated to 925 C for 15h in air reground and reheated at C in air for 36h with intermediate grinding

39 Nb based tetragonal tungsten bronzes Ba 0.4 Ca 0.2 Ti 0.2 Nb 0.8 O 3 Ba 0.6 Ti 0.2 Nb 0.8 O 3 Ba 0.4 Ca 0.2 Ti 0.2 Nb 0.8 O 3 Conductivity and dependencies on PO2 in (a) low and (b) high PO2

40 (AA ) 0.6 Ti 0.2 Nb 0.8 O 3 Nb based tetragonal tungsten bronzes Ba 0.4 Ca 0.2 Sr 0.6 Ba 0.4 Sr 0.2 Ba 0.2 Sr 0.43 Ba 0.6-x A x Ti 0.2 Nb 0.8 O 3 (A = Sr, Ca) materials appear the most encouraging as potential anodes They are synthesised in air and are stable also in reducing conditions It is possible to regenerate the electrical properties of anodes (leak in the FC) by rereducing the sample Ba 0.4 Ca 0.2 Sr 0.6 Ba 0.6 Ba 0.4 Sr 0.2 Ba 0.2 Sr 0.4 Ba 0.6 Log conductivity vs log PO2 in (a) low and (b) high PO2

41 Nb based tetragonal tungsten bronzes XRD for Ba 0.6 Mn Nb O 3, the pattern corresponds to that expected for a tetragonal tungsten bronze with no additional peaks present. Of the (Ba/Sr/Ca/La) 0.6 M x Nb 1-x O 3- δ (M = Mg, Ni, Mn, Cr, Fe, In, Sn) only the samples with M = Mg, In are of further interest as potential anodes The other samples are not sufficiently stable vs decomposition in low p (O2).

42 Nb based tetragonal tungsten bronzes The sample show a p(o2) -1/4 dependence for the conductivity The observed dependence can be obtained by assuming the oxygen vacancies being effectively constant due to the presence of a large number of inherent oxygen vacancies Ba volatilization; Cation vacancies

43 Layered Perovskites, La 2 Sr n-2 Ti n O 3n+1 End members: La 2 Ti 2 O 7 SrTiO 3 Perovskite slabs joined by crystallographic shears where the excess oxygen is accommodated. Potential oxygen ion or proton conductor due to the significant amount of interstitial oxygen found in both reduced and oxidised forms. Partial removal of the excess oxygen by reduction of Ti 4+ might lead to an enhancement of the ionic conductivity together with electronic conductivity due to the presence of Ti 3+

44 Layered Perovskites, La 2 Sr n-2 Ti n O 3n+1, n = 6 member La 2 Sr 4 Ti 6 O 19-δ Wet Ar Ea = 1.0 ev Dry Ar Ea = 0.3 ev A pronounced dependence of the total conductivity (i.e. grain and grain boundary) with the oxygen partial pressure features typical of an n-type conductor, (higher conductivity at lower oxygen partial pressure) Air-Total Ea = 1.3 ev Arrhenius plots for La 2 Sr 4 Ti 6 O 19-δ in air, wet Ar and dry Ar. Air-Bulk Ea = 0.8 ev E a decreases as the P (O2) decreased (from 1.3 ev in air to 0.3 ev in dry argon): Ti 4+ Ti Ti 3+ more reduced the conditions > Ti 3+ > electronic conductivity. No evidence of ionic conduction

45 Layered Perovskites, La 2 Sr n-2 Ti n O 3n+1, n = 6 member La 2 Sr 4 Ti 6 O 19-δ Nyquist plot measured in dry Ar Two semicircles: grain boundary and electrode response Similar responses in different atmospheres (wet Ar, static air) At higher temperatures, the electrode response is less important and above 300 C only the grain boundary can be observed. Complex impedance plots for measured in dry Ar.

46 La 2 Sr 4 Ti 6 O 19-δ : Polarization resistance Impedance measurements at 850 C R p decreases with the increase in temperature R p in wet CH 4 is almost three times larger than in wet H 2 : La 2 Sr 4 Ti 6 O 19-δ is not a suitable anode material for direct methane fuel cells 97% H 2 3% H 2 O 900 C 97% H 2 3% H 2 O 850 C 4.9% H 2 2.3% H 2 O 92.8% Ar 900 C 4.9% H 2 2.3% H 2 O 92.8% Ar 850 C 97% CH 4 3% H 2 O 900 C Fuel cell performance using La 2 Sr 4 Ti 6 O 19-δ as anode, La 0.8 Sr 0.2 MnO 3 as cathode, YSZ as electrolyte.

47 La 4 Sr 8 Ti 12-x M x O 38-δ : Disruption of extended defects oxide anode formed from lanthanum-substituted strontium titanate (La-SrTiO 3 ) in which the oxygen stoichiometry is controlled in order to break down the extended defect intergrowth regions and create phases with considerable disordered oxygen defects. Ti substituted by Ga and Mn to induce redox activity and allow more flexible coordination Anode powder by solid state reaction from La 2 O 3, SrCO 3, TiO 2, Mn 2 O 3 and Ga 2 O 3 fired for h. Polarization measurements in a three-electrode arrangement. Electrolyte = sintered 8 mol% Y 2 O 3 stabilized ZrO 2 Cathode = La 0.8 Sr 0.2 MnO 3 The anode was prepared in two configurations: first as a µm-thick layer of 50:50 LSTMG:YSZ 2. Four layers, with graded concentration of YSZ. Each layer pre-fired at 300 C and all of them co-fired at 1200 C for 2 h.

48 The lower members n < 7, are layered phases, having oxygen rich planes in the form of crystallographic shears joining consecutive blocks Increasing n (=11), planes become more sporadic with increasing n (= decreasing oxygen content) until they are no longer crystallographic features, rendering local oxygen-rich defects randomly distributed Oxygen excess parameter (δ) critically determines whether defects are ordered or disordered with δ = being a critical parameter (La 4 Sr n-4 )Ti n O 3n+2 a c, HRTEM images of samples varying from disordered extended defects (a, n = 12) through random layers of extended defects (b, n = 8) to ordered extended planar oxygen excess defects (c, n = 5). Substitution of Ti 4+ by Nb 4+ or Sc 3+ = influence on δ Ti inflexibility in coordination demands

49 La 4 Sr 8 Ti 12-x M x O 38-δ : Stability La 4 Sr 8 Ti 11 Mn 0.5 Ga 0.5 O 37.5 forms as a single-phase perovskite (monoclinic) on firing at 1400 C. No chemical reactions were observed by XRD on firing an intimate mixture of LSTMG and YSZ pressed powder at 1200 C in air for 80 h: good chemical compatibility. The phase is stable under fuel conditions at 1000 C; The perovskite structure is retained Electrode interface. SEM image, showing the cross-section of a fuel cell after testing.

50 La 4 Sr 8 Ti 12-x M x O 38-δ : Performance Dopants: to make the B-site co-ordination more flexible and to improve electro-catalytic performance the most successful = a combination of Mn and Ga. Mn supports p-type conduction in oxidizing conditions, and has been shown to promote electro-reduction under SOFC conditions Mn is known to accept lower coordination numbers in perovskites, especially for Mn 3+, and thus it may facilitate oxide-ion migration. Ga is well known to adopt lower co-ordination than octahedral in perovskite-related oxides.

51 La 4 Sr 8 Ti 12-x M x O 38-δ : Performance wet H 2 wet CH 4 wet CH 4 wet H 2 Polarization measurements on LSTMG/YSZ with varying temperatures and atmospheres. Fuel cell performance plots for different fuel gas compositions E is potential difference between electrodes, j is current density and P is power density.

52 MIEC double-perovskite system: Sr 2 MgMoO 6 based systems 1) The perovskite structure can support oxide-ion vacancies to give good oxide-ion conduction 2) A perovskite containing a mixed-valent cation from the 4d or 5d block can provide good electronic conduction 3) The ability of Mo(VI) and Mo(V) to form molybdyl ions allows a sixfoldcoordinated Mo(VI) to accept an electron while losing an oxide ligand = catalytic activity 4) The use of the Mo(VI)/Mo(V) couple as the catalytic agent in a perovskite requires a double perovskite with an M(II) partner ion to balance the charge 5) If the two octahedral-site cations of the double perovskite are each stable in less than sixfold oxygen coordination, the perovskite structure can remain stable on the partial removal of oxygen.

53 Layered Double Perovskites A 2 BB O 6-δ, where A is normally Sr, and B is Mo The most widely studied: SrMgMoO 6 Unit cell of NdBaCo 2 O 6 δ (for orthorhombic structures, the O(3) site splits into O(3) and O(4)). The key features: 1. B and B are ordered in alternate corner-shared octahedra Substitution at A or B sites can alter the cation valence and oxygen- vacancy concentration. Mg ions show unchanged divalence; only the valence of Mo ions changes from +6 to +5 with the introduction of oxygen vacancies. 2. High electronic conductivity (above the metal-insulator transition temperature - around 350 C) 3. Excellent oxide ion conductivity

54 Co and Ni containing Mo based double perovskites Co Ni Since the Mo(VI)/Mo(V) redox couple is at a higher energy than the M(III)/M(II) couples, reduction of the samples will, first, reduce the M(III) to M(II) The percentages of Co(III)/Co and Ni(III)/Ni in the as-prepared Sr 2 CoMoO 6 and Sr 2 NiMoO 6 samples sintered in air were 6.7% and 4.2%, respectively. Cation reduction in H 2, CH 4. Ni Power density and cell voltage as functions of current density at 800 C in H 2, dry CH 4, and wet CH 4 for (A) Sr 2 CoMoO 6 and (B) Sr 2 NiMoO 6 Co

55 La 1-x A x CrO 3 materials 1. Interconnect material 2. Reasonable electronic conductivity 3. Stability at elevated temperatures under both oxidizing and reducing conditions.

56 La 0.65 Ce 0.1 Sr 0.25 Cr 0.5 Mn 0.5 O 3 Improved performance in CH 4 (La/Sr) 1-x Cr 0.5 Mn 0.5 O 3-δ (0<x<0.1) Pr, Sr Ce Mn La 1-x A x CrO 3 Stable at elevated temperature in oxidizing and reducing conditions (p-type σ=20-35 Scm -1 in oxidizing conditions, and 1-3 S cm -1 in reducing conditions). Pr 0.7 Sr 0.3 Cr 0.9 Ni 0.1 O 3-δ Redox stable anode, with conductivities of 27 and 1.4 S cm -1 at 900 C in air and 5% H 2, respectively The oxide ion conductivity is still relatively low The catalytic activity is also relatively low. Ni Ni (4%) added to (La/Sr) 1-x (Cr/Mn)O 3-δ Ni introduces additional catalytic performance, The low levels used appear to avoid problems of C formation.